1. Field of the Invention
The present invention relates to an electron storage ring, which may, for example, form part of an apparatus for generating synchrotron radiation.
2. Description of the prior art
It is known to generate synchrotron radiation using an electron storage ring. As shown in FIG. 1 of the accompanying drawings, electrons are generated and accelerated by a linear accelerator 100 and fed to a syncrotron 101 where they are further accelerated. After suitable acceleration, the electrons, which now form a beam, are fed to an electron storage ring 102. That ring comprises a plurality of bending magnets 1, a plurality of quadrupole magnets 2, and may further include sextupole magnets 3. The electron storage ring 102 stores the beam of electrons, and the deflection of the beam at the bending magnets 1 generates synchrotron radiation which is passed down suitable conduits 103 to e.g. an inspection site 104.
Depending on the energy of the beam, which is partially affected by the size of the system, the synchrotron radiation may be used for many different functions. At relatively low energies, the beam may be used in, for example, the manufacture of semiconductor devices, while at higher energies, the main applications are in materials science.
FIG. 2 of the accompanying drawings shows a detail of part of the electron storage ring 102 of FIG. 1, and illustrates the relative locations of the deflection magnets 1, the quadrupole magnets 2, and the sextupole magnets 3. FIG. 2 also shows a radio-frequency acceleration cavity 10 which is used to accelerate further the beam, which passes in an equilibrium orbit 20.
One key parameter of the synchrotron radiation generated from the electron storage ring is its brightness. (intensity). In order to maximize this, it is desirable for the beam to be as concentrated as possible, i.e. its transverse dimensions should be as small as possible. These dimensions are determined by what is known in the art as the "emittance" of the beam, with beam size being proportional to the square root of the emittance.
The emittance of the beam in the electron storage ring is determined by the equilibrium relationship between the excitation of the radiation and radiation damping of betatron oscillations (oscillations centering round an equilibrium orbit in a direction perpendicular to the orbital axis of the beam), which damping ocurrs upon generation of synchrotron radiation. For a given electron beam energy, the emittance depends on the physical arrangement of the magnets forming the storage ring, but also on their excitation magnitudes which determine their field strength.
If the storage ring is constructed only of deflection magnets (which deflect the orbit around the ring) and quadrupole magnets, (which converge the beam orbit in the horizontal and vertical direction) then there are only double-pole and quadrupole components in the magnetic fields affecting the beam. The equation defining the betatron oscillations of the electron beam then becomes linear, and the beam is stable provided that there is an oscillation solution for the beam. If electron collisions are neglected (which collisions may occur due to e.g. dust or other material in the beam duct), the linearity of the equation is approximately maintained even when the amplitude of the beta oscillations is considerably larger than the beam duct, so that the beam is stable around the ring. Thus, it is possible to say that the dynamic aperture of the stable region of the beam is considerably larger than the physical aperture of the beam duct in which the beam passes.
However, with only deflection magnets and quadrupole magnets, the energy-dependency (chromaticity) of the beta oscillation frequency may depart from a substantially zero value, in which case the betatron oscillation frequency exhibits energy-dependency. In this case, the beam undergoes a head-tail instability due to lateral electron magnetic forces caused by electromagnetic fields (wake fields) which occur due to the electron magnetic interaction between a group of electrodes and a vacuum conductor wall. As a result, heavy beam losses can arise. With only deflection and quadrupole magnets, the chromaticity assumes a positive or negative value (always negative in large-size rings) and this is undesirable.
Therefore, in order to make the chromaticity substantially zero, sextupole magnets are provided at the places where the energy dispersion function is large. Thus, a head-tail instability can be avoided, but there is a side effect, namely that the dynamic aperture is reduced. The reason for this is that the sextupole magnetic field components give rise to an amplitude-dependency in the betatron oscillation frequency. Thus, if the amplitude becomes large, the betatron oscillations undergo a thirdorder resonance, and at still larger amplitudes stable oscillation solutions disappear.
Therefore, in order to increase the brightness of the beam, the chromaticity correction required becomes larger, and therefore stronger sextupole fields are needed. However, this has the effect of reducing the dynamic aperture of the beam.
There is, however, a practical problem with reduction in the dynamic apperture of the beam. When a packet of electrons is injected into a ring already containing a beam, the procedure of injection is as follows. Suppose that an electron beam is already stored in the storage ring 102, and it is wanted to add energy (i.e. more electrons) to that beam. Those electrons are accelerated by the linear accelerator 100, further accelerated by the synchrotron 101, and then transferred to the storage ring. Use is made of a septum magnet which deflects the injected electrons into a path substantially parallel to the main beam, which main beam is itself displaced towards the septum magnet. Subsequently, both the main beam and the newly injected electrons are moved sideways, in a direction so that the main beam moves away, from the septum to a position in which the newly injected electrons are within the septum, and also within the dynamic aperture of the beam. In this position the newly injected electrons and the beam will merge.
However, it can be appreciated that this process depends on the dynamic aperture of the beam being sufficient to include both the main beam and the newly injected electrons when the beam is moved sideways. Thus, the dynamic aperture must have a minimum radius in the direction that the beam is moved which is given by the sum of half the stored beam size, the effective thickness of the septum, and full size of the beam of new electrons to be injected. This is the minimum since errors and operational inefficiencies must be allowed for.
Therefore, if the dynamic aperture of the beam is too small, injection of new electrons becomes difficult or impossible.
Therefore, the dynamic aperture must be maintained sufficiently large to permit injection, which leads to increased emittance, and hence to increased beam size which limits the brightness of the synchrotron radiation.
Attempts have been made to solve this problem, but none have proved wholly successful. It is known from, for example "IEEE Particle Accelerator Conference Number 1 (1987) pp 443-445" to enlarge the dynamic aperture with the emittance maintained low, and to provide further sextupole magnets, in addition to those for correcting chromaticity, at positions where the energy dispersion function is zero.
This has the problem that the number of harmonic sextupole magnets are magnets are increased, and that the gain in dynamic aperture is small so that the corresponding gain in brightness is not great.
It is also known to make use of two storage rings, the beam being built up to a predetermined amount in one ring, at a high emittance, with the beam then being transfered to a low emittance storage ring by a one-turn on axis injection. In this way, the dynamic aperture of the second storage ring may be small, so that the emittance is low. Such a proposal is discussed in "Nuclear Instruments and Methods in Physical Research A246 (1986), pp 4-11". This method has, however, the grave disadvantage that two electron storage rings are needed, which increase the cost of the system significantly.